The process industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties.
In typical processing plants, a communication bus, such as a 4-20 mA current loop is used to power the process variable transmitter. Examples of such current loops include a FOUNDATION(trademark) Fieldbus connection or a connection in accordance with the Highway Addressable Remote Transducer (HART) communication protocol. In transmitters powered by a two-wire loop, power must be kept low to comply with intrinsic safety requirements.
A process temperature transmitter provides an output related to a sensed process substance temperature. The temperature transmitter output can be communicated over the loop to a control room, or the output can be communicated to another process device such that the process can be monitored and controlled. In order to monitor a process temperature, the transmitter includes a sensor, such as a resistance temperature device (RTD) or a thermocouple.
An RTD changes resistance in response to a change in temperature. By measuring the resistance of the RTD, temperature can be calculated. Such resistance measurement is generally accomplished by passing a known current through the RTD, and measuring the associated voltage developed across the RTD.
A thermocouple provides a voltage in response to a temperature change. The Seebeck Effect provides that dissimilar metal junctions create voltage due to the union of the dissimilar metals in a temperature gradient condition. Thus, the voltage measured across the thermocouple will relate to the temperature of the thermocouple.
As temperature sensors age, their accuracy tends to degrade until the sensor ultimately fails. However, small degradations in the output from the sensor are difficult to detect and to separate from actual changes in the measured temperature. In the past, temperature transmitters have used two temperature sensors to detect sensor degradation. If the output from the two sensors is not in agreement, the temperature transmitter can provide an error output. However, this technique is not able to detect a degradation in the sensor output if both of the two temperature sensors degrade at the same rate and in the same manner.
One technique which has been used in situations in which power is not a constraint is described in U.S. Pat. Nos. 5,713,668 and 5,887,978, issued Feb. 3, 1998 and Mar. 30, 1999, respectively, to Lunghofer et al. and entitled xe2x80x9cSELF-VERIFYING TEMPERATURE SENSORxe2x80x9d each of which is herein incorporated fully by reference. These references describe a temperature sensor having multiple outputs. The multiple outputs all vary as functions of temperature. However, the relationships between the various outputs and temperature are not the same. Further, the various elements in the temperature sensor change over time at differing rates, and in differing manners and react differently to various types of failures. A computer monitors the output from the sensor using a multiplexer. The computer places data points from the sensor into a matrix. By monitoring the various entries in the matrix and detecting changes in the various element or elements of the matrix relative to other elements, the computer provides a xe2x80x9cconfidence levelxe2x80x9d output for the measured temperature. If the confidence level exceeds a threshold, an alarm can be provided.
However, the art of low power process variable transmitters has an ongoing need for improved temperature sensors such as those which provide improved accuracy or a diagnostic output indicative of the condition of the temperature sensor.
A two-wire temperature transmitter is coupleable to a two-wire process control loop for measuring a process temperature. The transmitter includes an analog to digital converter configured to provide digital output in response to an analog input. A two-wire loop communicator is configured to couple to the process control loop and send information on the loop. A microprocessor is coupled to the digital output and configured to send temperature related information on the process control loop with the two-wire loop communicator. A power supply is configured to completely power the two-wire temperature transmitter with power from the two-wire process control loop. A temperature sensor comprises at least two temperature sensitive elements having element outputs which degrade in accordance with different degradation characteristics. The element outputs are provided to the analog to digital converter, such that the microprocessor calculates temperature related information as a function of at least one element output from a first temperature sensitive element and at least as a function of one degradation characteristic of a second temperature sensitive element.